Down syndrome (DS) is the leading genetic cause of intellectual disability. Individuals with DS struggle with all facets of expressive language, including narrative, a form of spoken language that enables a speaker to relate personal experiences or fictional stories to a listener. Well-developed narratives include a rich network of mental state language that serves as a pro-social way to communicate one's thoughts, feelings, and desires and as a forum for discussing others' perspectives and expressing empathy, all crucial to positive social interaction. Individuals with DS exhibit limited use of mental state language relative to chronological age and developmental level expectations, which affects the quality of their narratives and ability to interact with peers. Problems in using mental state language may contribute to the difficulties that individuals with DS experience with peer relationships and friendships in adolescence and with gaining meaningful employment in adulthood. Enhancing mental state language use, within a broader intervention designed to promote narrative competence and social communication, is likely to contribute to improved quality of life across the lifespan for individuals with DS. Developing such interventions requires a better understanding of how mental state language use develops in this population and the identification of the factors that shape its development. Accordingly, we propose to (1) investigate the developmental trajectory of mental state language in children with DS relative to various domains of development (i.e., age, nonverbal cognition, and expressive language), and (2) evaluate parent and child predictors of mental state language use in children with DS. These data will provide insight into the constraints on mental state language development in DS and serve as a first step toward identifying the underlying mechanisms that support or inhibit this behavior. Based on models from typical development, maternal use of mental state language and child emotion knowledge (recognizing and labeling emotion expressions and their causes/consequences) will be examined as concurrent predictors of child mental state language in DS. Maternal and child mental state language will be measured during narrations of wordless picture books, a common parent-child interaction context. Child emotion knowledge will be measured through direct assessment of children's abilities to recognize another child's dynamic emotion expressions from facial and contextual cues. After determining the independent contributions of each predictor, we will examine their relative contributions to child mental state language. The data generated by this study will set the stage for future longitudinal and experimental research to firmly establish causal pathways for the development of mental state language use in children with DS. Ultimately, this research will lead to targeted interventions to enhance mental state language use and thereby promote narrative and social-communicative competence in this population.